Liquid developer for electrostatic images



United States Patent US. Cl. 252-62.1 5 Claims ABSTRACT OF THE DISCLOSURE A liquid developer for latent electrostatic images is disclosed wherein the clumping together of pigment particles from larger aggregates is avoided by including a resin that is soluble in the carrier liquid and is selected to be a wetting agent for the pigment particles which thereby coats said particles.

T his invention relates to an improved liquid developer for electrostatic images.

In the art of liquid development of electrostatic images it is well known to use a carrier liquid of high electrical resistivity and to suspend in this the toner particles which are then attracted from the carrier liquid by the electrostatic force and are deposited image-wise.

It is also known to provide a fixing medium for such toner particles, such for instance as a resin or similar material which is in a dissolved or softened state and which deposits with the toner particles or as a coating on the toner particles to fix the image on evaporation of the carrier liquid.

It is known also to provide control substances for the toner particles which coat the toner particles and cause a change in the electrical characteristic of the particles.

It will be realized that in effecting development of electrostati images it is necessary for the particles to have a charge, and in this connection it is now well known that when particles are suspended in a liquid they have a natural charge which can itself be used to effect movement of the particles in an electrostatic field or which can be modified as said by providing a second material preferably coated on to the particles which modifies the electrical charge of the particles.

Problems which exist in liquid development are the provision of materials which will not deteriorate on storage and at the same time will not bleed away the charge from an image area at too rapid a rate and so prevent effective development of the electrostatic images, a further problem being the fixing of the toner particles to the area on which they are deposited.

Possibly one of the greatest problems with liquid development is to ensure adequate keeping qualities of the developer. Obviously developers should have a relatively long shelf life so that they can be held in storage in their final form or can be used in printing machines or the like without having to constantly stir the developer or circulate it or even change the developer.

It is therefore an object of this invention to provide an improved liquid developer for electrostatic images in which the suspension of the particles of pigment material will be ensured and in which clumping together of particles to form larger aggregates will be avoided, which effect, if it did occur, would cause sedimentation and coarser printing.

It is a further object to provide a stable developer which will have ready fixing properties of the particles to the image to give a scuff-free image.

The objects have been achieved according to our invention by suspending pigment particles in the developer carrier liquid and to also carry in this developer liquid a resin which is so selected that there is only a very low attraction between the developer particles and the resin, with the result that only a thin coating of resin can adhere to the developer particles, the bulk of the resin remaining in solution in the carrier liquid and acting as a means to cause the particles to remain in an isolated condition without undue tendency of sedimentation or agglomeration.

The invention thus consists of a liquid developer of the type utilizing pigment particles suspended in a carrier liquid having relatively high electrical insulating properties, which carrier liquid also supports a resin, characterized in that the particles have a diameter of not greater than one micron and are insoluble in the carrier liquid, and in that the resin is soluble in the carrier liquid and is selected to be a wetting agent for the particles which, however, coats the particles so thinly that the normal particle charge in the carrier liquid is substantially unaltered, and in that the resin is present in such a quantity in relation to the particles that there are approximately six parts of resin to one part by weight of pigment but wherein only about two percent of the said resin in relation to the weight of the pigment is capable of being held on to the pigment. Because of this the pigment particles are forced apart to deposit independently when subjected to an electrostatic field.

When the above conditions are observed it has been found that automatic fixing of the image takes place due partly to the size of the pigment particles but due mainly to the isolation of the pigment particles from each other to avoid agglomerates which would then be too large for effective fixing.

The resin acts as a dispersing agent which enables the pigment particles to remain in suspension for long periods without settlement or change of physical properties.

Development can take place readily from such liquid and it is found that the particles are deposited on the surface in a manner where they will be automatically fixed to the surface partly, as stated earlier because of the fact that they are extremely fine, and because of the presence of resin which will be co-deposited on evaporation of the carrier liquid in which the resin is dissolved, the scufffree nature of the developed image resulting mainly from the use of particles which are independently deposited and are too small to shear when a scuffing action is applied and thus maintain their original form and position, It will be realized that the small particles which deposit independently, because of the minute nature of the particles, can be held to the surface much more readily. Also small individual particles can be bonded better by means of resin.

As stated earlier the quantity of the resin must be suflicient to satisfy the surface of the toner particles but to leave most of it in solution in the carrier liquid, the amount of resin which is over in relation to the carrier liquid serving in part to provide more adequate fixing and in part to control the sensitivity and speed of the developer because normally the resin will have a lower volume resistivity than the carrier liquid, so that when more free resin is present in the solution the overall resistivity of the liquid component of the developer will be lowered with consequent faster destruction of the electrostatic image and thus with a change in the contrast of the image which is developed.

It is advantageous to cure the resin, that is substantially fully polymerize it, so as to reduce the oxygen require- 3 ments of the resins, because if this is done it is found that overprinting is possible which might otherwise be a problem.

It has been found that when the oxygen requirement of the particles is reduced by curing, a second electrostatic image can be built up and developed on a previously developed surface, this feature being of substantial importance when effecting color printing where a plurality of colors must be overprinted.

EXAMPLE I grams of Irgalite Fast Brilliant Blue G.L.S. pigment, was mixed with 60 grams of a long oil linseed oil modified alkyd resin known under the trade mark PC2/ 70 of the Polymer Corp. Pty. Ltd., Sydney, oil length 72%, and was bar milled in the presence of grams of xylol.

Irgalite is the trade mark of Geige for a phthalocyanine blue pigment.

2 grams of the concentrate so produced was then dispersed in 250 grams of Shellite" by stirring.

Shellite is the trade mark of the Shell Company for a hydro-carbon solvent of relatively high electrical resistivity.

The developer resulting from this was then ready for use and was found to have a positive polarity.

The fixing properties and the stability were found to be excellent and fill-in was also very good.

The developer has an ionization current of 1.9 microamperes at 10 kv. which is equal to 0.076 microamperes per square centimetre at 4 kv. per centimetre.

EXAMPLE II 10 grams of Irgalite Fast Brilliant Blue was mixed with 60 grams of Paralac 10 in the presence of 40 grams of xylol and the concentrate was bar milled and then dispersed by placing 2 grams of the concentrate into 250 grams of mineral turpentine.

Paralac 10 is the trade mark of Imperial Chemical Industries for a long oil linseed oil modified alkyd resin, oil length 72%.

This developer again had a positive polarity with good stability and fixing properties and excellent fill-in.

The ionization current was found to be 0.51 microamperes at 10 kv. and the ionization current density is 0.0204 microamperes per centimetre at 4 kv. per centimetre field.

EXAMPLE III 10 grams of carbon pigment were bar milled with 24 grams of Super Beckosol 1352 resin (an isophthalic long oil alkyd by Reichold Chemical Ind. Ltd., of U.S.A.) in the presence of 50 grams of perchloroethylene, oil length 60%.

After milling, 1 gram of the concentrate was dispersed in 500 grams of perchloroethylene, the resultant developer being found to have a negative polarity with good stability and fixing properties but a lesser amount of fill-in of heavier areas, but this developer was found to be more 4 effective in developing images which were produced by methods which do not involve charging of the photoconductor prior or after completing the electrostatic pattern by means of a light image or X-ray image.

The ionization current was found to be 2.1 microamperes at 10 kv. which equals an ionization current density of 0.084 microamperes per square centimetre at a field of 4 kv. per cubic centimetre.

What we claim is:

1. An improved liquid developer consisting essentially of pigment particles and a carrier liquid, said pigment particles being suspended in said carrier liquid, said liquid having relatively high electrical insulating properties, and a resin in said carrier liquid and having an oil length of not less than about and being substantially fully polymerized, the pigment particles having a di ameter of not greater than one micron and being insoluble in the carrier liquid, the resin being soluble in the carrier liquid, there being approximately six parts of resin to one part of pigment by weight, the said resin being a wetting agent for the pigment particles and being such that only about two percent of the said resin in relation to the weight of the pigment is held to the pigment, whereby the pigment particles are forced apart to deposit independently when subjected to an electrostatic field.

2. An improved liquid developer according to claim 1 wherein the resin in the developer provides for an ionization current density of between about 0.076 microgrripere per square centimeter at 4 kv. per centimeter 3. A method for preparing an improved liquid developer comprising mixing about 10 grams of pigment with about 40 to 60 grams of a modified alkyd resin having an oil length of not less than about 60% and adding suf ficient solvent to allow milling to form a concentrate, effecting said milling to form said concentrate, and suspending two parts of the said concentrate in about grams of an electrically insulating hydrocarbon carrier liquid which is also a solvent for the resin.

4. A method according to claim 3, wherein the alkyd resin is soluble in turpentine and wherein the said concentrate is suspended in about 100 grams of turpentine.

5. A method according to claim 3, wherein the resin is an isophthalic alkyd resin soluble in perchloroethylene and wherein the said concentrate is suspended in about 100 grams of perchloroethylene.

References Cited UNITED STATES PATENTS JOHN D. WELSH, Primary Examiner US. Cl. X.R. 

